The invention relates to a method of removing sulfur oxides from flue gases by absorption in seawater which is then treated so that the seawater may be returned to the recipient without damage to the marine environment.
The principle of using seawater as absorbent for acid gases, like SO.sub.2, is not novel. The Electrolytic Zinc Company of Tasmania has used seawater in order to remove sulfur dioxide from melting plant flue gases since 1949 (F. H. C. Kelly, Proc. Aust. Inst. Min. Metall, N.J. Nos. 152-153, 17-39 (1949). Design and Operation of a Counter Current Gas Scrubbing System). Absorption takes place by contacting gas and liquid e.g. in a so called packed counter-current tower. Seawater is well suited for absorbing SO.sub.2 due to the natural alkalinity of the seawater. Normally, seawater has a pH of from about 8.0 to 8.3 and an alkalinity of from 2.2 to 2.4 milliequivalents/liter (meq./1). By the absorption of SO.sub.2 the acid addition is buffered thereby that the equilibria: EQU CO.sub.2 (g).revreaction.CO.sub.2 (1) (1) EQU CO.sub.2 (1)+H.sub.2 O.revreaction.HCO.sub.3.sup.- +H.sup.+( 2) EQU HCO.sub.3.sup.- .revreaction.CO.sub.3.sup.2- +H.sup.+ ( 3)
move to the left.
In natural seawater most of the carbon dioxide is present as bicarbonate, and the alkalinity of the seawater essentially derives therefrom that the bicarbonate may be transferred into CO.sub.2 (1) according to equation 2.
SO.sub.2 is present in the liquid as SO.sub.2 (1), HSO.sub.3.sup.- and SO.sub.3.sup.2- dependent upon the equilibria: EQU SO.sub.2 (g).revreaction.SO.sub.2 (1) (4) EQU SO.sub.2 (1)+H.sub.2 O.revreaction.HSO.sub.3.sup.- +H.sup.+( 5) EQU HSO.sub.3.sup.- .revreaction.SO.sub.3.sup.2- +H.sup.+ ( 6)
If oxygen is dissolved in the liquid, the sulfite ion will react with oxygen in agreement with equation 7. EQU SO.sub.3.sup.2- +1/2O.sub.2 (1).fwdarw.SO.sub.4.sup.2- ( 7)
This implies that effluent consisting of untreated seawater used as absorbent for SO.sub.2 will consume some of the oxygen content of the recipient.
The oxidation reaction has been studied in several scientific works. The reaction takes place in several stages, and equation 7 only discloses the gross reaction. The rate of reaction increases with increasing pH and with increasing sulfite concentration whereas the oxygen concentration usually has little influence on the rate of reaction.
The proportion of dissolved sulfur dioxide which is present as sulfite increases with increasing pH. For the same total sulfur dioxide concentration the sulfite concentration increases with increasing pH and, accordingly, pH is a very important parameter.
The reaction (7) is catalyzed by polyvalent cations, inter alia Fe, Mn, Cu and Co. The reaction is also catalyzed by one or more of the intermediate products of the reaction.
The effect of an oxygen-demanding and acid effluent will depend upon several circumstances, like local recipient conditions, wind and current conditions, the concentration levels of the effluent, the magnitude of the effluent, the effluent arrangement and upon possible treatment of the seawater before it is discharged.
At the above-mentioned plant in Tasmania acid seawater is discharged from the scrubbers to the recipient in the untreated state.
At Bankside Power Station in England water from the river Thames is used in order to absorb SO.sub.2 from the flue gas (Litter, A. Central Electricity Generating Board, "Flue Gas Washing at Power Stations in the U.K.", July 1976). Approximately 1/10 of the cooling water which is used in the turbine condensers is pumped to the gas scrubbers. Limestone is added in order to increase the absorptive capacity of the liquid. The liquid from the scrubbers flows through a settling tank in order to remove particulate material. The liquid is then introduced into an aeration tank wherein sulfite ions are oxidized to sulfate ions (equation 7). Manganese sulfate is added as catalyst. The residence time of the liquid in the aeration tank is about 7 minutes. Following the aeration tank the liquid has a pH of about 2.3 and contains SO.sub.2 in an amount of about 44 mg/l which corresponds to an oxygen consumption of about 11 mg O.sub.2 /l.
The scrubbing water is then mixed with the rest of the cooling water before it is again discharged into the Thames.
At Bankside the pH of the effluent from the aeration tank is as low as 2.3. The pH of the liquid which enters the tank is higher, however, during the oxidation the relatively weak acid, sulfurous acid, is in principle transformed into the strong acid, sulfuric acid, with consequential lowering of pH. Accordingly, the reaction is slow and it is necessary with a reaction period of about 7 minutes in the aeration tank and a significant catalyst addition. Both for economic and spacial reasons it is desirable to reduce the residence time, i.e. to reduce the size of the oxidation apparatus.
In U.S. Pat. No. 4,085,194 a process is disclosed for removing SO.sub.2 from waste gases by contacting the gas with seawater in a gas scrubber, e.g. seawater which previously has been used as cooling water in a thermal power plant. Seawater is used in a such amount that the amount of sulfur dioxide absorbed by the seawater is significantly smaller than the total alkali equivalent of the seawater. This implies that for a power plant combusting coal containing from 1.0 to 1.5% S the entire amount of cooling water from the turbine condensers must be used in the gas scrubber.
The SO.sub.2 -containing seawater is then contacted with an oxygen-containing gas for decarbonating the seawater in order to increase the pH of the seawater to the neutral range, pH of from 6 to 7, and in order to oxidize sulfite ions to sulfate ions.
When acidifying seawater the equilibria according to the above-mentioned equations 1, 2 and 3 are displaced to the left whereby also the partial pressure of CO.sub.2 above the solution increases. The partial pressure of CO.sub.2 increases most strongly in the pH range of from 8 and down to 5. The partial pressure at pH 2 is then 1.1 times higher than at pH 5 and 200 times higher than at pH 8.
When contacting acidified seawater with e.g. air CO.sub.2 is transferred from the seawater to the air, so called decarbonation. This implies that the equilibria according to equations 1, 2 and 3 are further displaced towards the left, i.e. that H.sup.+ -ions are consumed with increasing pH.
If seawater is used in such an amount that the amount of sulfur dioxide which is absorbed in the seawater is substantially smaller than the total alkali equivalent of the seawater, a pH increase due to decarbonation may superimpose the lowering of pH due to the oxidation reaction, as disclosed in said U.S. patent specification.
It is a disadvantage of the described process that it presupposes that large amounts of seawater are to be used both in the absorption unit and in the decarbonation/oxidation unit. This implies an increase of the process equipment and high operation costs. It might be desirable to discharge the treated seawater into deep water by means of diffusers in order to obtain a rapid dilution and to avoid influencing the biologically productive surface layer. However, this is very expensive when large amounts of liquid are to be discharged.
Flue gases normally contain from 10 to 15% by volume of CO.sub.2. Some CO.sub.2 will be absorbed in the gas scrubber. When increasing the ratio between liquid and gas in the scrubber higher amounts of CO.sub.2 will be absorbed, and there will then also be higher amounts of CO.sub.2 which have to be removed in the decarbonation/oxidation unit. This increases the required size of the unit.
In an article in International Journal of Sulfur Chemistry (L. A. Bromley, Int. J. Sulfur Chemistry, Part B, Vol 7, Number 1 (1972) a process of desulfurizing flue gases from power plants is disclosed. The process is relatively equal to the process disclosed in the above-mentioned U.S. patent specification. In the gas scrubber amounts of seawater are used corresponding to the amount of cooling water for the power plant. The pH of the seawater as it leaves the gas scrubber will normally be about 6. However, it is suggested that limestone, dolomite or other forms of alkali may be added if the seawater leaving the gas scrubber is too acidic.
In the so called Flakt-Hydro process (brochure published in October 1978 by A/S Norsk Viftefabrikk, Oslo, Norway, "The Flakt-Hydro Process Sulfur Dioxide Removal by Seawater") only about 1/5 of the amount of cooling water from a thermal power plant is normally used in the gas scrubber for absorption of SO.sub.2. The SO.sub.2 -containing seawater is subsequent to the absorption mixed with the remainder of the amount of cooling water before the entire amount is introduced into a decarbonation/oxidation unit.
The above-mentioned seawater processes have in common that the natural alkalinity of the seawater is essentially used in order to absorb and neutralize the content of sulfur dioxide of the flue gas.
However, in most known processes for the desulfurization of flue gas the purification is carried out using a slurry of lime or limestone in fresh water as absorbent.
With these processes operational trouble arises in the gas scrubber due to scale formation causes by precipitation of calcium carbonate, calcium sulfite and calcium sulfate. These purification processes yield as end product more or less water-containing calcium sulfite and/or calcium sulfate for which a deposition site might be difficult to find at several locations. Below a so called "lime/gypsum" process will be described (B. Colliander, "Status Report per April 1, 1978 concerning desulfurization of flue gas. Prepared for the coal consequence board in Denmark") in which the suspension, also often called "the slurry", from the scrubbing tower is supplied to an oxidation unit in which gypsum is formed according to equations 9, 10 and 11. EQU CaSO.sub.3 (s).revreaction.Ca.sup.2+ +SO.sub.3.sup.2- ( 9) EQU SO.sub.3.sup.2- +1/2O.sub.2 .fwdarw.SO.sub.4.sup.2- ( 10) EQU SO.sub.4.sup.2- +2H.sub.2 O+Ca.sup.2+ .fwdarw.CaSO.sub.4.2H.sub.2 O(s) (11)
The gypsum is removed and the solution returned to the gas scrubbers. The pH value of the suspensions from the gas scrubber is kept at 4.0-5.0. The pH value is then adjusted to 3.5-4.5 by the addition of H.sub.2 SO.sub.4 in order to promote the oxidation in the subsequent process step. The sulfur dioxide is essentially present as undissolved calcium sulfite in the liquid from the gas scrubber. This means that undissolved calcium sulfite must be dissolved before the oxidation reaction can take place.
It has been mentioned above that a high pH gives increased rate of oxidation. In this system the solution of calcium sulfite is determining for the total rate of reaction and, accordingly, pH must be lowered to 3.5-4.5 in order to promote the dissolution of the calcium sulfite. Thus, the pH is maintained at a level at which the oxidation reaction is slow and, accordingly, it is necessary with long residence times for the liquid in the oxidation unit.
In the advent of the present invention extensive investigations have been made in order to provide a method for absorbing SO.sub.2 in seawater with subsequent treatment of the SO.sub.2 -containing seawater, whereby
(1) the essential portion of the excess of acid in the SO.sub.2 -containing seawater is reduced by the addition of calcium based alkali in the form of CaO, Ca(OH).sub.2 or CaCO.sub.3.
(2) An essential portion of the content of sulfur dioxide in the seawater is transformed into sulfate by the addition of an oxygen-containing gas.
(3) The pH of the liquid is maintained at a favourable level both with regard to the oxidation reaction and for expelling CO.sub.2 from the liquid.
The amount of seawater for the absorption and the subsequent treatment may be kept relatively small according to the present method. Thereby the decarbonation/oxidation reactor and a possible effluent arrangement will be significantly smaller than for the seawater processes described. The end product from the SO.sub.2 purification process according to the invention is seawater which may be returned to the recipient without risk for the marine environment. The invention is of particular interest in connection with coal or oil fired power plants and correspondingly fired industrial boilers. However, the invention is not restricted to use in connection with one particular industry.